This invention relates generally to the removal of resist layers and more particularly to a method of dissolving positive photoresists which are based on novolak resins.
Photoresists are mainly required for photolithographic processes. In these processes, substrates are etched selectively. In order to permit this, the substrates are covered with a mask that is resistant to the etchant and which leaves exposed only those parts of the substrate which are to be etched. For making such a mask, the substrate is first covered with a continuous photoresist layer. After the drying of the photoresist, the applied layer is exposed to actinic radiation through a mask which, if a positive photoresist is used, shows the same pattern as the photoresist mask to be made. Positive photoresist has the characteristic feature that it becomes soluble upon exposure. When the photoresist layer is subjected to a suitable solvent after exposure, the previously exposed areas are dissolved, i.e. in other words, the photoresist layer is developed, to leave an etchant-resistant patterned mask of unexposed photoresist. After etching, the photoresist mask is no longer required and is stripped off. Photolithographic processes have reached considerable importance, particularly for the making of printed and integrated circuits. An essential criterion for the quality of a photolithographic process is the dimensional fidelity with which a respective pattern is etched. The growing microminiaturization and continuously increasing packing density of the components in integrated circuits is possible only because photolithographic processes have been continuously improved. Because the demands made on photolithographic processes are particularly high in semiconductor technology, the process described here is presented by means of procedural developments used in semiconductor technology. It should be pointed out, however, that the process described here can be employed generally in photolithography.
Below, "photoresist" always refers to "positive photoresists which contain alkali soluble, phenol-formaldehyde novolak resins", even if this precise wording is not used. These resists contain, in addition to the resin, a sensitizer such as a diazo ketone, for example, a naphthoquinone (1, 2)-diazide sulfonic acid ester as described in U.S. Pat. Nos. 3,201,239 and 3,666,473.
Those procedural steps in photolithography where the photoresist is partly or fully removed, i.e., the development of the photoresist mask and its removal after etching, are among the decisive factors for the usefulness of a photolithographic process. For developing patterns of novolak resin based positive photoresists, basic developers are generally used. Such developers cannot dissolve photoresist layers which have been baked. Baking decisively influences the resist adhesion on the substrate, the resist adhesion improving with rising temperature. Inferior resist adhesion causes undercutting. Good resist adhesion is therefore a condition for high dimensional fidelity of the etched pattern. High dimensional fidelity of the etched patterns becomes increasingly important, with rising microminiaturization in the manufacture of integrated circuits, and consequently the demands made on resist adhesion increase, too. The photoresist mask can, it is true, also be baked after developing, but since photoresist structures start to flow at temperatures higher than 140.degree. C, which affects the dimensional fidelity of the photoresist mask and thus of the etched patterns, baking after developing has to remain below this temperature and, therefore, optimum resist adhesion cannot be achieved. The known development of the photolithographic process, as it has to take place owing to the basic developers, therefore, presents unsatisfactory results for patterns with small dimensions and high packing density.
When stripping the photoresist mask after etching, it is of essential importance to remove the photoresist without leaving any residues, and without affecting the substrate. A dry process is known where organic material is burned in a glow discharge. The process reliably removes photoresist, but it is time-consuming and involves a great amount of apparatus, and there is the danger that ions are generated by electron bombardement in the oxide layers. This cannot be tolerated particularly when making field effect transistor components. Known processes for the wet chemical removal of positive photoresists also have problems with respect to one or more of the following: expense, removal, disposal, process conditions and hazards and attack or contamination of the substrate.